1. Field of the Invention
The present invention generally relates to a digital multimedia broadcasting (DMB) system, and more particularly, to a conditional access system (CAS) for DMB.
2. Description of the Related Art
Generally, a conditional access system (CAS) is a system for ensuring that pay broadcasts are only accessible to those who are entitled to watch them. The CAS performs a function for limiting access to services such that subscribers can receive the services only when paying viewing fees. That is, the CAS is the technology enabling “pay services” to be provided. When the broadcast content is scrambled and transmitted receivers are permitted to receive broadcast content only if viewing fees are paid.
FIG. 1 illustrates an exemplary structure of a digital multimedia broadcasting (DMB) system in which the CAS is implemented. Referring to FIG. 1, a DMB station 100 scrambles compressed audio, video and data streams using subscriber information stored in the DMB system. An entitlement control message (ECM) indicating conditional access features for specific broadcast content or a specific broadcast channel and a message indicating a subscriber's entitlement based on the subscriber information (i.e., a subscriber-by-subscriber entitlement management message (EMM)) are multiplexed along with the scrambled streams. Then, a plurality of stream packets (hereinafter, referred to as transport stream (TS) packets) are sent to a DMB satellite 104.
Then, the DMB satellite 104 sends the TS packets to a DMB receiving terminal 102 on the ground. The DMB receiving terminal 102 receives and demultiplexes the scrambled TS packets from the DMB satellite 104 and extracts the ECM from the TS packets. The DMB receiving terminal 102 detects the presence of conditional access features for the specific broadcast content or the specific broadcast channel from the extracted ECM, and checks the EMM to determine if the reception of the broadcast content for the subscriber is limited through the subscriber information stored in the DMB receiving terminal 102. If it is determined that the subscriber is entitled to receive the broadcast content using the EMM, the DMB receiving terminal 102 descrambles the scrambled TS packets, such that a user can watch DMB. However, if it is determined that the subscriber is not entitled to receive the specific broadcast content using the EMM, the DMB receiving terminal 102 does not descramble the scrambled TS packets, such that the user cannot watch the specific broadcast content.
Because the broadcast content is collectively broadcast to DMB receiving terminals on the ground in case of DMB using the DMB satellite, a spillover phenomenon occurs in which the DMB is provided to an undesired region. When the DMB system broadcasts any broadcast content or broadcast channel limited to a specific region, TS packets may be received from the DMB satellite 104 regardless of a region. There is a problem in that the conventional DMB system cannot limit the reception of specific broadcast content in a specific region.
The ECM indicating conditional access features for specific broadcast content and the EMM indicating if the subscriber is entitled to receive the broadcast content can limit the reception of the specific broadcast content for the subscriber. These are only provided as means for limiting the reception of the broadcast content itself as in pay broadcast content or parental ratings for the broadcast content. However, the EMM and ECM do not provide regional conditional access features according to location of the DMB receiving terminal. There is a problem in that limiting the reception of specific broadcast content or a specific broadcast channel only in a specific region is impossible or various service fee systems cannot be provided to users according to DMB reception regions of the users.